Electronic devices are generally interconnected to each other, in order to accomplish a number of tasks. An example is that of a Multi-Chip Module (MCM), wherein a plurality of circuits integrated in corresponding chips of semiconductor materials are packaged in a single electronic assembly. Another example is that of a probe card, which is used to contact integrated circuits at a wafer level for their test.
Several solutions have been proposed to achieve the desired result. Particularly, a specific technique known in the art is based on the raising of flexible leads.
For example, EP-A-0352020, which is incorporated by reference discloses a system for interconnecting multiple chips by means of a semiconductor carrier. For this purpose, conductive pads of each chip are connected to corresponding textured portions of conductive pads provided on the carrier (facing to each other). In order to increase the mechanical compliance of the structure so obtained, EP-A-0352020 teaches to arrange a localized layer of insulating material between each chip and a portion of its pads; the insulating material is selected so as to have relatively little or no adhesion with the chip pads. The chip is pressed against the carrier, thereby connecting the portions of the chip pads resting on the insulating material to the corresponding carrier pads; the chip is then slightly pulled up to space it apart from the carrier (for example, by 2 mm). In this way, the chip pads detach from the insulating layer, thereby extending them between the chip and the carrier (with the insulating material that can also be removed at the end). The structure proposed in EP-A-0352020 allows withstanding strains caused by mechanical or thermal stresses.
A variation of the same technique is proposed in EP-A-0870325, which is incorporated by reference. In this case, a removable layer is exploited to facilitate the raising of the leads. More specifically, the leads are formed over a multi-layer sheet (consisting of a dielectric sheet sandwiched between two metal layers); each lead is shaped as a strip, which extends between a tip end and a terminal end. The metal layer under the leads is then etched, so as to separate the strips from the dielectric sheet. The tip end of each lead is instead slightly larger than its strip, so that the etching process leaves a small button under it; this button provides a very small adhesion of the tip end to the dielectric layer (just strong enough to retain the tip end against gravitational and acceleration forces in normal handling). On the other hand, the terminal end of each lead is far larger, so that the same etching process leaves a bigger button under it; this button firmly secures the terminal end to the multi-layer sheet (at the same time connecting the lead—through a via-hole—to a corresponding terminal being formed on its opposed surface). The component so obtained is aligned with a wafer, and the tip ends are bonded to corresponding contacts of the wafer. As above, the multi-layer sheet and the wafer are spaced apart, so as to extend the leads among them (with the tip ends of the leads that readily detach from the multi-layer sheet).
In any case, the leads used in interconnection elements are generally protected by a dielectric material (preferably of the elastic type). For example, U.S. Pat. No. 3,795,037, which is incorporated by reference discloses a connector with resilient leads, which are embedded in an elastomeric material; the structure so obtained allows connecting electronic devices, without requiring any accurate control of the eight of the leads. The connector is produced by defining the leads in a series of frames (for example, by chemical milling). A stack formed by multiple frames with interposed spacers is then build up, and clamped between two plates. At this point, an elastomeric liquid is injected into the cavity defined by the plates, and it is cured; at the end, the plates are removed so as to obtain the desired structure.
The same technique is also applied in the above-mentioned document EP-A-0870325. In this case (after the leads have been extended), a flowable material is injected between the multi-layer sheet and the wafer—to fill the available space and to penetrate among all the leads. As above, the material is then cured so as to embed the leads in an elastic dielectric layer.
However, the solutions known in the art may not be completely satisfactory. Indeed, these techniques are relatively complex; for example, they require the use of sacrifical layers that adversely affect the corresponding manufacturing processes.
Moreover, it may be very difficult to obtain an acceptable level of quality. For example, in the structure disclosed in EP-A-0352020 the chip pads may easily detach from the insulating material (before their connection to the carrier). On the other hand, a very high accuracy is typically required in EP-A-0870325 to obtain the correct size of the buttons under the tip ends; indeed, the corresponding manufacturing process must typically be perfectly controlled to ensure that the tip ends are retained by the multi-layer sheet (before their connection to the wafer), but at the same time they readily detach when the leads must be extended.